Plasmonic resonances in finite-size two-dimensional conductors are able to couple strongly with THz radiation and can be tuned by application of a gate voltage, as presented in Ju Long, et al., “Graphene Plasmonics for Tunable Terahertz Metamaterials”, Nature Nanotec., 6, 630-634 (2011), and Yan Hugen, et al., “Tunable Infrared Plasmonic Devices using Graphene/Insulator Stacks”, Nature Nanotec., 7, 330-345 (2012). These features are attractive for a variety of electrically tunable terahertz devices, such as, for example, detectors, emitters, filters, and modulators.
Among various two-dimensional systems, graphene is a particularly attractive plasmonic medium due to its electrical and thermal properties at room temperature. Particularly promising is terahertz (THz) photodetection, in which graphene-based devices may offer significant advantages over existing technology in terms of speed and sensitivity.
Due to graphene's low electronic heat capacity and relatively large electron-electron relaxation rate compared to its electron-phonon relaxation rate, hot electron effects are prominent in graphene even at room temperatures. The hot electron effects have been exploited to attain fast and sensitive THz detection via the photothermoelectric effect and bolometric effect.
Observed plasmonic resonances occur in isolated graphene elements, where the isolating charge accumulates at opposite edges of a sub-wavelength graphene element. Integrating such a plasmonic element with an antenna, metamaterial, or electrical contact greatly increases the range of potential applications. However, a conductive boundary inhibits the accumulation of charge at the edges which was previously thought to prevent the plasmonic resonance.
In addition, a significant challenge remains in increasing graphene's absorption. Graphene's interband absorption is determined through a frequency-independent constant πα≈2.3%, where α is a fine structure constant. Owing to its zero band gap nature, doped graphene shows a relatively high DC activity, resulting in a considerable Drude absorption (free carrier response) in the THz range. However, the Drude absorption in graphene is strongly frequency dependent, decreasing by (ωτ)−2 degrees at high frequency ω>>1/τ, where τ is a scattering time, proportional to graphene's mobility, and typically ranges between 10 fs and 100 fs in graphene. Thus, the Drude absorption rolls off at the lower frequencies in high mobility graphene samples.
Among its many outstanding properties, graphene supports terahertz two-dimensional plasma waves: subwavelength charge density oscillations connected with electromagnetic fields that are tightly localized near the graphene sheet. When these waves are confined to finite-sized graphene, plasmon resonances emerge that are characterized by alternating charge accumulation at the opposing edges of the graphene. The resonant frequency of such a structure depends on both the size and the surface charge density and can be electrostatically tuned throughout the terahertz range by applying a gate voltage.
Graphene plasmons have been explored or proposed for use in biosensors, terahertz detectors, terahertz emitters, and a growing number of devices in the nascent field of terahertz optoelectronics. It is increasingly recognized that graphene holds the potential for filling a critical gap in terahertz technology.
The promise of tunable graphene THz plasmonics has yet to be fulfilled, because most proposed optoelectronic devices require near total modulation of the absorption or transmission, and need antenna coupling or electrical contacts to the graphene. Such constraints are difficult to meet using existing plasmonic structures. Until now, there was no experimental evidence that two-dimensional plasmons could be confined with conductive boundaries.
A number of efforts have been made to increase the absorption in graphene photodetectors. For example, quantum dots have been deposited on graphene to enhance the light-scattering direction. However, this approach is limited to the visible or near infrared (where the interband absorption of the quantum dot lies), and the response times are unacceptably slow.
Another approach contemplates placing a photodetector in a micro cavity, which resonates at a selected frequency. This approach can enhance absorption, but to date this has been demonstrated only at near-infrared wavelengths and can be cumbersome for long wavelength THz radiation.
Coupling the detector to an antenna is a viable approach for frequencies up to the low THz, but there are few demonstrations of antenna-coupled graphene devices, and the approach is applicable only to devices whose size is much smaller than the wavelength.
It is therefore desirable to overcome the deficiency of the prior approaches in pursuit to achieve a strong absorption and attain improved operational parameters in graphene-based optoelectronic systems.